Revealed: The private lives of electrons

Using laser light, scientists have looked into the complex relationship
that develops between a single electron and its environment, known as a Kondo state.
The break-through could aid the development of quantum computers.

Scientists at Princeton University have brought fresh ideas to
the study of the Kondo problem, a phenomenon first observed in the 1930s when researchers
were surprised to find that resistance to electricity flowing through certain metals
increases at very low temperatures. Normally, resistance through metals decreases
as temperature is lowered.
A tangled relationship

Thirty years later, Japanese scientist Jun Kondo explained the
phenomenon as resulting from the presence of cobalt or other magnetic impurities
in the metals. Other scientists further realized that the Kondo effect resulted
from a relationship between electrons known as entanglement, in which the quantum
state of one electron is tied to the quantum states of neighboring electrons, even
if the particles are later separated by considerable distances. In the case of the
Kondo effect, a trapped electron is entangled in a complex manner with a cloud of
surrounding electrons.

Intrigued by this effect, researchers set out to understand how
a trapped electron becomes entangled with its environment. They hoped the findings
could help overcome barriers to quantum computing.

Although previous observation methods allowed scientists to make
measurements of the Kondo state, they could not provide information on how electrons
developed such a relationship with their surroundings. To better understand how
electrons become entangled, an international team of scientists from the US, Germany
and Switzerland investigated the idea using a laser to probe electrons that evolve
into the Kondo state. The investigators developed a theory about how laser light
scattered off electrons could carry information about this process.

They discovered that, depending upon the state of the electron,
it should absorb various colors of laser light to varying degrees. The light reflected
back would carry a signature of the entangled quantum state, offering a window into
the relationship between the trapped electron and its environment. They used nanostructured
devices to isolate the electrons and tested their theory by projecting a laser beam
on the device and measuring the transmitted light.

The light signature matched theoretical predictions, and the researchers
discovered that they could use it to confirm when they had turned off the Kondo
state using a magnetic field.

The findings, which appeared in Nature (doi: 10.1038/nature10204),
could allow new ways of storing and processing information or threaten to destabilize
the computing process.